The present invention relates to a circular machine for knitting or hosiery with a device for tensioning the knitted fabric.
Knitted fabric tensioning devices for large- and medium-diameter single-cylinder circular machines, for tensioning the knitted fabric that is formed continuously with a considerable length, are known. These tensioning devices generally comprise two or three tensioning rollers which are arranged so that their axes are horizontal and face each other. The tensioning rollers are arranged below the needle work area of the machine and the fabric formed by the needles passes between said tensioning rollers. At least one of the tensioning rollers is rotated about its own axis so as to tension the fabric that is gradually formed by the needles and passes between the tensioning rollers. The fabric that leaves the tensioning rollers is then wound onto a roll which is arranged below the tensioning rollers in the footing of the machine.
The electric motor that drives at least one of the tensioning rollers, in these conventional tensioning devices, is usually driven with a torque which corresponds to the chosen tension degree. A potentiometer is installed on the power supply circuit of the electric motor and allows to vary the torque of the motor, thus varying the degree of tension of the fabric in order to adapt it to the various production requirements.
Some problems have been observed in the use of these tensioning devices.
A first problem is the difficulty in determining the size of the reduction unit that connects the electric motor to at least one of the tensioning rollers. In order to achieve an adequate degree of tension while using a compact electric motor with a modest power rating, it is necessary to use a very high reduction ratio. Consequently, transmission of the motion from the motor to the tensioning roller entails high inertia in achieving a variation in the degree of tension of the fabric when the tensioning requirements related to the current production change.
Moreover, with conventional tensioning devices the degree of tension of the fabric cannot be checked in order to ensure that it actually corresponds to the intended tension degree.